Hydrogen is an important feedstock in the manufacture of ammonia, methanol, and a variety of other chemicals; but its largest market is the crude oil processing industry. In crude oil refineries, hydrogen is used in a number of processes including hydrodesulfurization where hydrogen is reacted with sulfur containing compounds over a catalyst to form hydrogen sulfide. Hydrogen sulfide itself is already produced in great quantities during the drilling and processing of natural gas and oil. A process that can economically extract hydrogen from low value feedstocks or wastes such as hydrogen sulfide would bring tremendous benefits to the petroleum sector as this sector consumes large amounts of hydrogen.
Many processes exist for the production of hydrogen. The production of hydrogen is currently dominated by the steam reforming process where a relatively light hydrocarbon is reacted with steam inside a bed of reforming catalyst, usually nickel. Since steam reforming of hydrocarbon is endothermic, the energy to drive the reactions must be provided from an external source. In the steam reforming process, the hydrocarbon-containing stream must be free of sulfur or other contaminants such as carbon particles that can poison and deactivate the catalyst.
Another hydrogen production method is partial oxidation. In a partial oxidation reaction, a hydrogen-containing feed is reacted with an oxidizer, such as oxygen or air, in substoichiometric proportion normally referred to as a rich mixture where the equivalence ratio spans from one 1 to the upper flammability limit of the fuel being utilized as the feedstock. The equivalence ratio, defined as the stoichiometric oxidizer to fuel ratio divided by the actual oxidizer to fuel ratio, is shown in equation R1.
                    EquivalenceRatio        =                                            (                              fuel                Oxidizer                            )                        actual                    /                                    (                              fuel                Oxidizer                            )                        stoichimetry                                      R1      An equivalence ratio less than unity is considered lean, also referred to as fuel-lean, since a portion of the oxidizer is leftover after all of the fuel is consumed by the oxidation reaction. Where the fuel content of the mixture lies below the lower flammability limit of the fuel used as the feedstock, the fuel and oxidizer mixture is considered ultra-lean. Conversely, fuel and oxidizer mixtures of equivalence ratio greater than unity are considered rich, also referred to as fuel-rich, since a portion of the fuel is leftover after the oxidation reaction is complete. Mixtures of equivalence ratios greater than rich mixtures, normally taken to be higher than the upper flammability limit of the fuel being utilized as the feedstock, are considered ultra-rich. Ultra-rich mixtures do not normally produce self-sustained flames without the aid of external energy sources or preheating the mixture.
Although the partial oxidation process does not need an external source of heat since it is exothermic, it is still less common than steam reforming since it is generally less efficient than steam reforming particularly at large scale. As a normally non-catalytic process, partial oxidation can utilize any hydrocarbon feeds. The steam reforming and partial oxidation processes can be combined into a single process normally referred to as an autothermal process. In the autothermal process, the energy for the reforming reactions is provided by oxidizing a small portion of the fuel inside the bed of a reforming catalyst. Due to its catalytic nature, the autothermal process falls under the same constraints as the steam reforming process in that the catalyst bed is susceptible to poisoning and deactivation by sulfur, carbon, and other poisons in the feed stream. The hydrocarbon stream must be desulfurized in a first step prior to entering the autothermal reactor. During reforming, whether by the steam reforming or autothermal process, water must be provided in excess of the stoichiometric quantity to prevent carbon formation. Additionally, excessive temperature must be prevented in the reactions to avoid sintering the reforming catalyst. Steam reforming, partial oxidation, and the autothermal process are well known methods in the industry that are practiced on industrial scales.
The invention disclosed herein can be an economical process for producing hydrogen from hydrocarbons and various other hydrogen containing fuels. U.S. Pat. No. 6,517,771 to Li, incorporated herein by reference, disclosed a reverse flow inert porous media reactor for the purpose of heat-treating metals. Li limited the reactant stream to methane and oxygen or air, and the preheater to initiate the process is located inside the porous bed. Drayton et. al 27th, International Symposium on Combustion, 27, pp. 1361-1367, 1998, incorporated herein by reference, disclosed an application of the reverse flow reactor for fuel reforming, producing synthetic gas from methane in a reactor similar to Li's. None of the disclosed references above include an external energy source for the reverse flow reactor or are applied to the reformation of hydrogen sulfide.
A number of studies in reverse flow inert porous media reactors are carried out in applications not intended for hydrogen production from hydrocarbons. Hoffman et al, Combustion and Flame, 111, pp. 32-46, 1997, incorporated herein by reference, operated a reverse flow reactor with ultra-lean air and methane mixtures for the purpose of heating fluids. Barcellos et. al. Clean Air 2003, Seventh International Conference on Energy for a Clean Environment; Lisbon, Portugal, Jul. 7-10, 2003, incorporated herein by reference, tested a reactor similar to Hoffman's for the production of saturated steam through heat exchangers protruding directly through the inert porous media and fitted at the extremities of the reactor.
Production of hydrogen from both light and heavy hydrocarbons as well as other hydrogen containing wastes such as hydrogen sulfide is not addressed in the prior art. Hydrogen is a much more valuable commodity then sulfur. A process that can economically recover the hydrogen as well as other compounds could have significant impact on the petroleum and other industries. The reformation of hydrogen sulfide (H2S) to hydrogen and sulfur presents certain challenges not encountered in hydrocarbon reformation. For example, the low heat content of H2S precludes obtaining very high temperature in the partial oxidation regime. More importantly, H2S reforming requires the reaction to reach near equilibrium conditions at high temperature to obtain high yield. In the current invention, the intrinsic heat recuperating mechanism of the inert porous media matrix and the reactor's ability to create an isothermal high temperature volume render it a cost effective option for the reformation of H2S and other hydrocarbons by providing the necessary residence time and temperature without the requirement of an external energy source to be used continuously throughout the reactions.
Specifically, all of the reforming reactions in these above-mentioned prior art references occur inside a hollow chamber. None of these references disclose an apparatus and process where the reaction zone may be located in any portion of a reactor chamber, where the reaction zone is allowed to freely propagate through the reactor chamber filled with a porous media matrix and where the reforming reactions occur directly in a heated inert porous media matrix, or packed bed. Therefore, there has developed a need for a reactor which can efficiently reform both hydrocarbon and hydrogen sulfide fuels to pure hydrogen while not requiring continuous external energy to produce a viable hydrogen yield.